A diesel vehicle is recognized as a short-term solution of the CO2 problem due to its high power and high energy efficiency. However, large quantities of pollutants such as PM, CO, HC and NOx are contained in diesel engine exhaust gas, so they cause environmental pollution problems. Therefore, reducing apparatus of such pollutants is indispensable for completion of a clean diesel.
CO, HC and PM can be removed by installing cDPF at a rate of 90% or more. However, as for reduction of NOx, it is still under study.
NOx can be decomposed through the following reaction mechanism on a catalyst under existence of a reducing agent.4NO+4NH3+O2→4N2+6H2O,SCR  (1)NO+NO2+2NH3→2N2+3H2O,fast SCR  (2)6NO2+8NH3→7N2+12H2O,SCR  (3)
For reduction of nitrogen oxides, an exhaust gas purifying apparatus 10 is disclosed in the related art as illustrated in FIG. 1. The exhaust gas purifying apparatus includes a diesel oxidation catalyst (DOC) 200, a catalyzed diesel particulate filter (cDPF) 300 and a selective catalytic reduction (SCR) catalyst 400 installed sequentially at the rear end of an engine 100. Nitrogen oxides, CO, HC, and PM can be reduced by the exhaust gas purifying apparatus of such configuration.
According to the results of recent research, it is reported that it is possible to obtain a fast SCR of more than ten times when a reaction progresses on the path of reaction formula (2), as compared with reaction formula (1), in terms of kinetics of reaction (Manfred Koebel et al, Selective catalytic reduction of NO and NO2 at a low temperature, Catalysis Today, 73, 239-247(2002)). However, because the concentration of NO2 contained in exhaust gas of a car is less than 30% of the whole NOx, the level of contribution of self fast SCR progress is low.
That is, for SCR to progress through the path of reaction formula (2), a proper action is required so that the NO2 content in NOx can satisfy 50%. However, it is difficult to satisfy the NO2/NOx ratio in the gas supplied to the SCR catalytic layer by the configuration of FIG. 1, although NO2 is formed by the following reaction formula (4) under DOC. Therefore, it is necessary to improve the removal efficiency of nitrogen oxides.NO+½O2→NO2  (4)
The cause of the problem with such a NO2/NOx ratio is because NO2 formed through the path of reaction formula (4) in DOC goes on with PM collected in cDPF and reaction according to reaction formula (5).
That is, such reaction is desirable in terms of PM removal, but is not desirable in terms of nitrogen oxide reduction since it makes reverse action in achieving the fast SCR. It is because PM acts as a reducing agent of NO2 so that the ratio of NO2/NOx in exhaust gas introduced into the SCR is decreased depending on the PM collection quantity in the cDPF.NO2+PM(carbon)→NO+CO/CO2  (5)
For the purpose of improving the above-described problem, an exhaust gas purifying apparatus 20 is disclosed in the related art as illustrated in FIG. 2. The exhaust gas purifying apparatus uses a system of such configuration that can obtain a high NO2 concentration by arranging a DOC 200 downstream of a cDPF 300 and upstream of a SCR 400.
The above-described purifying apparatuses have no spatial clearance because they are installed on the lower body of a vehicle. In particular, because a change in a temperature of the exhaust gas is higher depending on the moving speed of vehicle, a maintenance fraction (time average) at 200° C. or more, which is the SCR action range, is no more than 50% based on small cars driving in downtown area.
That is, it is necessary to provide a post-treatment apparatus having such a configuration that, while a vehicle stops, exhaust gas at a temperature of more or less than 100° C. is supplied to cool a SCR catalytic layer, and when the vehicle departs and starts driving, a high-temperature exhaust gas is supplied thereto and used as a heat source to quickly raise the temperature of the SCR catalytic layer.
In addition, a SCR using an aqueous solution of urea as a reducing agent has its function decreased when hydrocarbon (HC) is contained in the exhaust gas. Therefore, it is essential that HC is removed from inflow gas. In particular, in case of stopping the vehicle, when a low-temperature exhaust gas contains hydrocarbon, that is, HC and the temperature of an oxidation catalyst in the purifying apparatus is low, oxidation reaction of HC is impossible, therefore, the exhaust gas containing HC not removed is supplied to the SCR catalytic layer. Therefore, it is not avoidable that HC is adsorbed in zeolite as a SCR catalyst. The SCR function cannot be exerted 100% until most of the exhaust gas is discharged by heating to and above a given temperature at which HC is released. Because of that, an apparatus with such a configuration that can suppress the supply of SCR in a low-temperature zone, is necessary.
Especially, a recent sharp rise in prices of precious metals acts as a limiting factor on use of precious metals over the whole industry. Therefore, in the field of automobile exhaust gas post-treatment which consumes large quantities of precious metals, it is absolutely necessary to minimize the consumption of precious metals along with functional performance satisfaction.